DEVELOPMENT AND CHARACTERIZATION OF HIGH PERFORMANCE AMMONIA BORANE BASED ROCKET PROPELLANTS

Abstract

Historically, hypergolic propellants have utilized fuels based on hydrazine and itsderivatives due to their good performance and short ignition delays with the commonly usedhypergolic oxidizers. However, these fuels are highly toxic and require special handlingprecautions for their use.In recent years, amine-boranes have begun receiving attention as potential alternatives tothese more conventional fuels. The simplest of these materials, ammonia borane (AB, NH3BH3)has been shown to be highly hypergolic with white fuming nitric acid (WFNA), with ignitiondelays as short as 0.6 milliseconds being observed under certain conditions. Additionally,thermochemical equilibrium calculations predict net gains in specific impulse when AB basedfuels are used in place of the more conventional hydrazine-based fuels. As such, AB may serve asa relatively less hazardous alternative to the more standard hypergolic fuels.Presented in this work are the results of five major research efforts that were undertakenwith the objective of developing high performance fuels based on ammonia borane as well ascharacterizing their combustion behavior. The first of these efforts was intended to bettercharacterize the ignition delay of ammonia borane with WFNA as well as investigate various fuelbinders for use with ammonia borane. Through these efforts, it was determined that Sylgard-184silicone elastomer produced properly curing fuel samples. Additionally, a particle size dependencywas observed for the neat material, with the finer particles resulting in ignition delays as short as0.6 milliseconds, some of the shortest ever reported for a hypergolic solid fuel with WFNA.The objective of the second area of research was intended to adapt and demonstrate atemperature measurement technique known as phosphor thermography for use with burning solidpropellants. Using this technique, the surface temperature of burning nitrocellulose (a homogeneous solid propellant) was successfully measured through a propellant flame. During thesteady burning period, average surface temperatures of 534 K were measured across the propellantsurface. These measured values were in good agreement with surface temperature measurementsobtained elsewhere with embedded thermocouples (T = 523 K). While not strictly related toammonia borane, this work demonstrated the applicability of this technique for use in studyingenergetic materials, setting the groundwork for future efforts to adapt this technique further tostudying the hypergolic ignition of ammonia borane.The third research area undertaken was to develop a novel high-speed multi-spectralimaging diagnostic for use in studying the ignition dynamics and flame structure of ammoniaborane. Using this technique, the spectral emissions from BO, BO2, HBO2, and the B-H stretchmode of ammonia borane (and its decomposition products) were selectively imaged and newinsights offered into the combustion behavior and hypergolic ignition dynamics of ammoniaborane. After the fuel and oxidizer came into contact, a gas evolution stage was observed toprecede ignition. During this gas evolution stage, emissions from HBO2 were observed, suggestingthat the formation of HBO2 at the AB-nitric acid interface may help drive the initial reactantdecomposition and thermal runaway that eventually results in ignition. After the nitric acid wasconsumed/dispersed, the AB samples began burning with the ambient air, forming a quasi-steadystate diffusion controlled flame. Emission intensity profiles measured as a function of height abovethe pellet revealed the BO/BO2-based emissions to be strongest in the flame zone (correspondingto the highest gas temperatures). Within the inner fuel-rich region of the flame, the HBO2 emissionintensity peaked closer to the fuel surface after which it unexpectedly began to decrease across theflame zone. This is seemingly in contradiction to the current understanding that HBO2 is a stable product species and may suggest that for this system it is consumed to form BO2 and other boron oxides.The fourth area of research undertaken during this broader research effort investigated theuse of ammonia borane and other amine borane additives on the ignition delay and predictedperformance of novel hypergolic fuels based on tetramethylethylenediamine (TMEDA). Despitethese materials being in some cases only sparingly soluble in TMEDA, solutions of ammoniaborane, ethylenediamine bisborane, or tetramethylethylenediamine bisborane in TMEDA resultedin reductions of the mean ignition delays of 43-51%. These ignition delay reductions coupled withthe significantly reduced toxicity of these fuels compared to the conventional hydrazine-basedhypergolic fuels make them promising, safer alternatives to the more standard hypergolic fuels.Attempts were made to improve these ignition delays further by gelling the TMEDA, allowing foramine borane loadings beyond their respective solubility limits. Moving to these higher loadingshad mixed results however, with the ignition delays of the AB/EDBB-based fuels increasingsignificantly with higher AB/EDBB loadings. The ignition delays of the TMEDABB-based fuelson the other hand decreased with increasing TMEDABB loadings, though the shortest were stillcomparable to those found with the saturated fuel solutions.The final research area that was undertaken was focused on scaling up and developing fuelformulations based on ammonia borane for use in a small-scale hypergolic hybrid rocket motor.Characterization of the regression rate behavior of these fuels under motor conditions suggestedthe fuel mass flow rate was driven primarily by the thermal decomposition of the ammonia borane.This mechanism is fundamentally different from that which governs the regression rate of mostconventional solid fuels used in hybrid rockets as well as that of ethylenediamine bisborane, asimilar material in the amine borane family of fuels. Understanding this governing mechanism further may allow for its exploitation to enable high, nearly constant fuel mass flow ratesindependent of oxidizer mass fluxes. If successful, this would enable further optimization of thedesign for rocket systems utilizing these fuels, resulting in levels of performance that rival that ofthe more conventional hydrazine-based fuels.